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The Age of Rocks

Perhaps you remember Spencer Tracy and Frederic March playing this scene in the movie Inherit the Wind. [1] Henry Drummond (the Clarence Darrow character) has called Matthew Harrison Brady (the William Jenning Bryant character) to the stand as an expert witness on the Bible—

Drummond: It’s sad that we don’t all have your positive knowledge of right and wrong, Mr. Brady. How old do you think this rock is?

Brady: I am more interested in the “Rock of Ages” than I am in the age of rocks.

Drummond: Dr. Paige of Oberlin College tells me this rock is at least 10 million years old.

Brady: Well, well, Colonel Drummond, you managed to speak here some of that scientific testimony, after all.

Drummond: Look, Mr. Brady. These are the fossil remains of a marine prehistoric creature found in this very county, and which lived here millions of years ago when these very mountain ranges were submerged in water.

Brady: I know. The Bible gives a fine account of the flood. But your Professor’s a little mixed up in his dates. That rock is not more than six thousand years old.

Drummond: How do ya know?

Brady: A fine biblical scholar, Bishop Usher, has determined for us the exact date and hour of the Creation. It occurred in the year 4004 B.C.

Drummond: Well, that’s Bishop Usher’s opinion.

Brady: It’s not an opinion. It’s a literal fact—which the good Bishop arrived at through careful computation of the ages of the prophets, as set down in the Old Testament. In fact, he determined that the Lord began the Creation on the 23rd of October, 4004 B.C. at, uh, 9:00am. [2]

I can “amen” the thought that we ultimately should be more interested in the Rock of Ages than the age of rocks, but the rocks are out there as part of the Creation, and their ages can tell us something about the age of the earth. Just how old are the rocks we find on Earth?

To cut to the chase, the most ancient rocks appear to be over 4 Gy old. That’s roughly a million times older than Bishop Usher’s date for creation. How do we know that they’re that old?

Geochronology is the science of determining the absolute age of rocks and sediments. A variety of dating methods are used by geologists to achieve this. These include radiometric techniques that measure the decay of radioactive isotopes and other radiogenic activity, incremental techniques that measure the regular addition of material to sediments or organisms, and correlation of various markers that allow age-equivalence to be established between different sites.

Radioactive decay is a statistically random process. We can’t predict when any particular atom will decay, but we can use statistics to predict what percentage of any sample will decay over a given period of time. Each isotope of every element has a known decay rate that is usually expressed as a half-life or the time during which half of the material will decay to another isotope.

Perhaps the best known form of radiometric dating is the use of carbon-14 (14C). Carbon-14 decays with a half-life of 5730 years to nitrogen-14. 14C dating is used to date organic material (e.g., plant-derived materials such as wood or paper), and can be applied to samples younger than about 50 ky.

Rocks are dated using other means. The oldest rocks are dated by the uranium-lead method. Uranium-lead is one of the oldest and most refined radiometric dating schemes with a routine age range of about 1 My to over 4.5 Gy and with a normal accuracy in the 1 percent range. The method relies on the coupled chronometers provided by the decay of uranium-238 (238U) to lead-206 (206Pb) (with a half-life of 4.46 Gy) and 235U to 207Pb (with a half-life of 704 My). One of the advantages of this process is the use of two separate, chemically identical chronometers. Any leakage of lead from the sample will result in a discrepancy between the two decay measurements, giving two different ages for the sample under test. This discordance provides a check on the reliability of the test.

Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4). Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Thus, the lead found in zircon is a trapped decay product. Undamaged zircon retains this lead generated by radioactive decay of uranium and thorium.

The oldest known materials found on Earth are zircons from the Canadian Shield and the Australian Outback. These zircons are around 4.4 Gy old.

We have an independent check on the reasonableness of these data.

For the last forty-odd years we’ve been launching spacecraft to Mars. So far, we have found that, while Mars no longer has an intrinsic magnetic field like the Earth’s, there is evidence that the planet’s crust has been magnetized, but that the planetary dynamo that drove the field died out around 4 Gy ago. We have found surface features on Mars that appear to be around 3.8 Gy old. These ages are more or less consistent with the time span of the earth’s geology so it’s reasonable to conclude that both planets were formed about the same time. 4.5 or 4.6 Gy is a good estimate for the age of the Earth.

Not all of rocks are as old as the Earth. Most of the rocks on the surface are much younger, having been deposited by various geological processes through time. Geochronology is the tool used to date the rock strata in various formations. It is used to date rocks and not fossils. Biostratigraphy is the science of assigning sedimentary rocks to a known geological period via describing, cataloging and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, it merely places the rock within an interval of time at which the fossil assemblage is known to have coexisted. Furthermore, it is only applicable to sedimentary formations and not to igneous rocks.

Geochronology is the prime tool used in the discipline of chronostratigraphy, which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies. Radiometric dating is the primary yardstick in geology. The fossil record is not.

The fossil record was one of the earliest sources of evidence for evolution. It is still a source of relevant data concerning the history of life on Earth. When Darwin wrote On The Origin of Species, the oldest know animal fossils dated to roughly 540 My ago. In the meantime, fossils as old as 3.5 Gy have been found. Most of these oldest fossils are microscopic, bacteria and the like.

Take a look at Figure 3 of this post. Note that the diagram shows that the DNA record predicts that birds are more closely related to reptiles than to mammals. That should lead us to predict that we should find fossils of some transitional common ancestors with both reptile and bird characteristics.

Figure 3 also leads us to predict transitional forms between reptiles and mammals. There are two significant differences in the basic bone structures of reptiles and mammals. First, reptiles have four bones in their lower jaw. Mammals have one. Second, reptiles have only one bone (the stapes) in the middle ear. Mammals have three. As the reptile fetus develops, two developing bones in the head eventually form two of the lower jaw bones. The same two bones in a mammalian fetus eventually from the hammer and the anvil bones of the middle ear. These are the two bones missing in reptiles. This would seem to indicate that if common descent be true, then one might expect some really weird transitional forms. One Young Earth Creationist explained the problem this way:

All mammals, living or fossil, have a single bone, the dentary, on each side of the lower jaw, and all mammals, living or fossil, have three auditory ossicles or ear bones, the malleus, incus and stapes. … Every reptile, living or fossil, however, has at least four bones in the lower jaw and only one auditory ossicle, the stapes. … There are no transitional fossil forms showing, for instance, three or two jawbones, or two ear bones. No one has explained yet, for that matter, how the transitional form would have managed to chew while his jaw was being unhinged and rearticulated, or how he would hear while dragging two of his jaw bones up into his ear. [3]

It turns out that this analysis is simply wrong. The actual transitions occurred over a long span of time so that the changes were gradual. In several known fossil intermediaries there are bones that seem to have overlapping functions, bones that could be called both ear and jaw bones. In some fossil species (Morganucodon, for example) the bones that would be the hammer and the anvil in a mammalian ear served as both ear and jaw bones simultaneously. Indeed, in modern reptiles these same bones transmit sound to the stapes in the middle ear. What we find in the fossil record is a process of these jaw bones becoming specialized in the hearing function and becoming a part of the middle ear.

Until a just a few years ago the fossil record was the primary source of information about evolution. Now it is becoming a backup for what we’re learning from genomics.

Facts are the world’s data. The world’s data shows us that any two objects will attract each other with a force directly proportional to their masses and inversely proportional to the square of the distance between them. That fact is the basis of Newton’s Theory of Universal Gravitation. The world’s data shows us that the Moon takes an elliptical path around the Earth with the earth at one focus of the ellipse and that the Earth has a similar path around the Sun. Those facts together with Newton’s theory disprove geocentric cosmology.

In Science something is factual when it has been confirmed to such a degree such that it is ridiculous to withhold provisional consent. Apples might start rising from the ground to the tree branch, but I wouldn’t bet on it. [4] The age of the Earth (and other planets for that matter), the fossil record, and our own DNA are facts. What we see in them tells us that evolution is a fact. Darwin emphasized the difference between his two great scientific accomplishments. One was establishing the fact of evolution. The other was proposing natural selection as a theory to explain how evolution occurred.

We haven’t worked out all the details of evolution, but the facts will not change while we refine our knowledge. Einstein’s work on gravity superseded Newton’s, but apples didn’t float in mid-air while Einstein’s theory was being tested.

If we believe in taking everything that God has said seriously, the fact of evolution from the evidence He has left in creation disproves certain interpretations of the Bible. Most obviously, it deflates a literal six-twenty-four-hour-days interpretation of Genesis 1.

As we’ll see in the next post, Young Earth Creationists are fixated on a literal interpretation of an English translation of Genesis 1, but not a literal interpretation of every Biblical description of creation. Here, for example, is God’s first-hand account:

Where wast thou when I laid the foundations of the
earth?
Declare, if thou hast understanding.
Who hath laid the measures thereof, if thou knowest?
Or who hath stretched the line upon it?
Whereupon are the foundations thereof fastened?
Or who laid the corner stone thereof;
when the morning stars sang together,
and all the sons of God shouted for joy? [5]

The literal cosmology of this passage implies that the Earth sits atop a foundation, and that implies a bottom surface resting on the foundation—which implies a flat Earth. [6] Yet, YEC adherents clearly believe that the Earth is spherical and moves in an orbit around the Sun. If one is willing to treat God’s eyewitness description of creation as poetic or figurative on the one hand, why must the Genesis account be treated literally on the other?

While everything the Bible tells us is true, it does not tell us everything that is true. Nor is everything in Scripture interpreted literally by everyone; otherwise, all Christians might hold a premillennial view of eschatology, and there would be no post- or amillennialists.

But eschatology is about last things. Let’s not get ahead of ourselves. We still have some more things about creation to consider.

NOTES

[1] The play contains a note reminding the reader that it is not history. It is, in fact, an extremely inaccurate dramatization of the Scopes Monkey Trial.